1. Field of the Invention
The present invention relates to diode pumped solid-state laser amplifiers. More specifically, the present invention relates to a laser disk folded amplifier architecture wherein a series stack of laser gain disks are oriented at an angle of incidence that is perpendicular or approximately perpendicular to the propagation direction of a laser beam having a predetermined wavelength.
2. Description of Related Art
Solid state-laser amplifier technology is a well-developed field wherein numerous modes of operation and embodiments have been demonstrated. One of the embodiments comprises conventional disk architecture wherein pump arrays (e.g., rows of flash lamps or diode-laser arrays) are situated in planes located on either side of one or more disk amplifiers. The disks themselves are tilted at Brewster's angle with respect to the laser beam. This is the angle at which a p-polarized laser beam experiences no reflection losses at the input and output surface of each amplifier disk. This approach was invented to scale solid state lasers to very large beam apertures, and it has served the world very well in large, single shot systems.
However, non-uniform pumping due to Brewster angle architecture generates deleterious wavefront distortions caused by the non-uniform distribution of waste heat from the optically pumping process. The result of such thermal gradients is bulk thermal deformation, an undesired change in the index of refraction, and stress in the material, all of which contribute to optical distortions of the transmitted wavefront of a laser beam to be amplified.
Several techniques have been utilized to mitigate the effects of thermal gradients during Brewster angle laser operation. First, diode pumping to match absorption lines of dopant ions in the gain materials of laser disks, reduces the amount of waste heat generated. Second, convective gas flow across the surfaces of the gain material can help dissipate heat-generated gradients. Background for such a method is described by Sutton, et al., in “Heat Removal in a Gas Cooled Solid-State Laser Disk Amplifier,” AIAA Journal, Vol. 30, No. 2, pp. 431-435, (1992). Another technique is to allow a laser gain medium to temporarily store the deposited heat. During laser operation, the active laser gain medium will heat up until it reaches some maximum acceptable temperature. The cooling cycle is then begun, in the absence of lasing, and elapsed time between periods of laser operation depends largely on the efficiency of the cooling of the laser during the suspended lasing action. Background for this concept is described and claimed in U.S. Pat. No. 5,526,372, issued Jun. 11, 1996 to Albrecht, et al., and assigned to the assignee of the instant application. Regardless of which technique is applied, thermal gradients that produce bulk thermal deformation, changes in the index of refraction, and stress due to non-uniform pumping of Brewster angle disk amplifier architectures continues to be a problem in high-average power solid-state laser systems.
The emergence of high average power diode arrays beyond the conventional technologies in which typically only a single laser diode bar was attached to a single high performance heat sink have enabled monolithic laser diode packages in which multiple diode bars are attached to a single high performance heat sink. This technology advance has led to larger laser diode arrays and larger diode-pumped laser systems that are capable of emitting pump light at nonzero emission angles, which may be utilized to solve current non-uniform optical pumping as discussed above. Background for one such type of package, which utilizes Silicon Monolithic Microchannels (i.e., SiMM) is described and claimed in U.S. Pat. No. 5,548,605 issued Aug. 20, 1996 to Benett, et al., U.S. Pat. No. 5,828,683 issued Oct. 27, 1998 to Freitas, and U.S. Pat. No. 5,923,481 issued Jul. 13, 1999 to Skidmore, et al., and assigned to the assignee of the instant application.
SiMM technology incorporates the formation of V-grooves for positioning and mounting of laser diode bars by Anistropic etching of silicon substrates. In <110> oriented silicon wafers, (the surface of the wafer is a <110> plane), etch rate differences can be exploited to etch channels that are perpendicular to the surface of the wafer. This is accomplished by creating a mask on the surface of the wafer that is aligned with the <111> planes on the wafer. When etched, these slow-etching, perpendicular <111> planes then become the walls of the channels. With the appropriate angular orientation of an etch mask on a <110> oriented silicon wafer, the result of the above etching method is to produce V-grooves, wherein laser emitting diodes or laser diode bars are attached to the slanted surfaces, i.e., the <111> plane, and as such are oriented to produce an emission direction in a very specific way relative to the <110> normal direction (e.g., 55 degrees).
SiMM arrays with a 55 degree emission angle or any diode array with a nonzero emission angle, measured from the normal to the array surface are useful in pump configurations that are integrated in normal incidence large aperture laser disk architectures. Such architectures provide better energy extraction efficiencies, better beam quality despite any residual thermal gradients in the laser gain disks, polarization independent extraction, and denser, compact system packaging.